This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Streptococcal Toxic Shock Syndrome (STSS) is a life-threatening disease caused by Group A Streptococcus (GAS) for which the virulence is mainly due to the presence of the surface coiled-coil M1 protein. The ability of GAS to invade host cells and cause STSS is dependent on the ability of M1 to bind human fibrinogen and subsequently induce vascular leakage. The individual structures of both components are already known but in order to understand the mechanisms of STSS at a molecular level, we propose to study the complex between M1 fragments and human fibrinogen using x-ray crystallography.